Many of the advantages gained through automated manufacturing are quickly lost when the speed of inspection is inadequate to keep pace with production. One important class of inspection procedures consists of dimensional measurements, for example measurement of the finish or geometry required in manufacturing of a surface. Finish refers to the roughness or small scale height variations of a surface, whereas geometry refers to the macroscopic characteristics of the surface. In-process inspection procedures based upon the measurement of such dimensions must be designed to survive in a comparatively harsh environment without sacrificing dynamic range, accuracy, or speed.
The inspection and quality control of drilled holes is of a special concern to certain industries, such as the aircraft industry. The diameter, finish and shape of holes drilled for fasteners in aircraft manufacturing must meet exceptionally high tolerances, because the fit of the fastener in the hole is critical to the strength and fatigue life of the joint. This is particularly true where the components are high-strength aluminum alloys, which are often fairly brittle and therefore notch sensitive. A commercial jet aircraft may contain a million drilled holes, and the inspection of all such holes by conventional techniques is impractical. Currently, inspection procedures typically rely on statistical inference based upon the inspection of a certain fraction of the holes. However, even such partial inspection is time consuming, and depends to a considerable extent upon the skill and experience of the inspector.
Prior surface finish measurement techniques may be divided into two classes: (1) those which measure or infer only average roughness and, (2) those which record the surface profile, i.e. the local height of the surface as a function of distance along the surface. Although often more difficult to obtain, the surface profile provides substantially more information than average roughness alone. Higher order statistics, such as skewness, kurtosis, and surface spectra can be computed from the profile record, but not from the value of the average roughness. In addition, profile measurements are usually necessary for reliable detection of individual cracks and tool gouges. The average roughness measurement from a surface containing an isolated surface defect is often indistinguishable from that of a surface containing uniform roughness of equivalent value.
The stylus-type profilometer is the device most commonly used to measure profile and average roughness. However, there are limitations to the in-process application of stylus profilometers. To measure profile, the stylus tip must mechanically follow the vertical height variations as the stylus is moved across the surface, and scanning speeds must therefore be relatively slow. In addition, the stylus tip suspension is fragile and is easily damaged.
Recently a variety of optical surface measurement techniques have been developed. These are noncontacting systems and have potentially high scanning speeds. Some are capable of profile measurements, as well as average roughness estimates. Disadvantages of these techniques are the potential complexity of the optical apparatus (light source, optics, alignment devices, detectors), and the possibility of errors due to contamination by cutting fluids and metal particles.
A pneumatic technique has also been used to measure surface finish. In this approach, a close-fitting sensor is placed against a machined surface and air is allowed to flow between the sensor and the surface. The pressure required to sustain a fixed flow rate is related to the characteristics of the surface finish. This method is limited to the measurement of average surface roughness, and has comparatively poor time response.
Known capacitive surface finish gauges use a wide, flexible metal electrode located parallel to and in close proximity with a conducting surface to be measured. The capacitance generated between these two conductors is inversely proportional to the average roughness of the surface. The sensor is rugged and the method is well suited to the manufacturing environment. However, only average roughness can be measured.
The most common hole geometry measurement system employed in industry is the air gauge. An air gauge consists of a close tolerance rod inserted inside the hole to be measured. Compressed air is then allowed to flow through small holes around the rod. By monitoring the airflow, the diameter of the hole can be determined. However, the response time of this system is slow, and hence it is not suitable for automated inspection systems.
There are other more exotic ways to measure the geometry of a hole, for example by means of strain gauges or by optical measurement. In both cases, the required apparatus is bulky, and the instruments perform best under laboratory conditions. A capacitive hole probe system has been developed that consists of a rod of approximately the size of the hole, with small capacitive plates attached along and around the axis of the probe. The hole wall serves as the opposing side of the capacitor plate. The capacitance created by each plate can then be translated into the distance between the probe and the hole at various circumferential locations. Unfortunately, such a probe can only measure the average surface roughness rather than the surface profile, and local scratches cannot be detected by such a system.